1. Field of the Invention
The present invention relates generally to the field of atomic frequency standards. In particular it relates to the class of standards generally known as passive atomic frequency standards in which the state of an atomic ensemble is prepared by means of optical pumping. More particularly, the invention is directed at using the coherence property of lasers, rather than their intensity, to accomplish the optical pumping and to implement a novel frequency standard with improved characteristics.
2. Description of the Prior Art
In some atomic frequency standards using alkali metal atoms, such as rubidium, optical pumping is used to prepare the atomic ensemble into a special state that allows the detection of the resonance signal which is used to lock a crystal oscillator to the transition F=1, m.sub.f =0 to F=2, m.sub.F =0 in the S.sub.1/2 ground state corresponding to a frequency nhf=6.835 GHz. These levels are identified in FIG. 1. The P state consists of two distinct levels P.sub.1/2 and P.sub.3/2 and transitions from the ground state S.sub.1/2 can take place to either of those levels. The corresponding radiations have wavelengths, 780 nm (to P.sub.3/2, termed the D.sub.2 radiation) and 794 nm (to P.sub.1/2, termed the D.sub.1 radiation). The technique consists in manipulating the atomic ensemble in such a way as to alter the distribution of atoms in the hyperfine levels of the ground state and is termed "state selection". In the passive rubidium 87 frequency standards presently available commercially the state selection is done by exposing the resonance cell containing the rubidium 87 atoms to the radiation of a properly filtered rubidium 87 spectral lamp. The filtering is done by means of a cell containing rubidium 85 which absorbs radiation corresponding to the transition from the ground level F=2 to the excited P state leaving a spectrum containing radiation which is resonant with the transition from the ground level F=1 to the excited P state. This filtering takes place for both radiation wavelengths D.sub.1 and D.sub.2. This is termed the separated filter approach (Packard et al., U.S. Pat. No. 3,129,389). The net effect is to populate the ground hyperfine level F=2 at the expense of the F=1 level. The process is termed population inversion. In some cases the filter is incorporated directly in the resonance cell through the use of natural rubidium which contains about 70% of rubidium 85 and 30% rubidium 87 (Jechart, U.S. Pat. No. 3,798,565). This is termed the integrated filter approach. These approaches create a so-called population inversion.
Recently, substantial efforts have been directed, with relative success, at the replacement of the spectral lamp with a solid state diode laser emitting at the proper wavelength, either D.sub.1 or D.sub.2, but tuned to one of the hyperfine ground states (Liberman et al., U.S. Pat. No. 5,670,914). In the case of cesium, the pumping can only be done with a laser since no isotopic hyperfine filtering of a Cs spectral lamp is possible, cesium having no other stable isotopes.
In these approaches a buffer gas, not chemically active with the alkali atoms, is used to restrain the motion of the alkali atoms, thereby limiting relaxation of the same atoms on the walls of the containing cell and also preventing broadening by Doppler effect. The buffer gases have a strong temperature coefficient that makes the resonance frequency sensitive to environmental fluctuations. This is generally avoided through the use of a mixture of buffer gases having opposing temperature coefficients, in combination with appropriate temperature regulation of the cell.
The resonance cell, containing the ensemble of alkali atoms, is placed inside a microwave cavity tuned to the transition between which the population inversion has been created. The light transmitted is detected with the help of a photodetector, as shown in FIG. 1. It is to be noted that, upon optical pumping, the cell becomes transparent to the incident radiation since atoms are pumped out of the absorbing level, F=1. Microwave energy is fed to the cavity and its effect on the atoms, when tuned to the hyperfine frequency, is to alter the population of the two levels of the ground state and, consequently, the optical transmission of the ensemble. The ground state hyperfine resonance signal is thus detected on the transmitted light and is used to lock the frequency of the microwave source used to feed the cavity. The resulting device is a system whose frequency is locked to an atomic resonance.
Although this approach has achieved substantial success, it has, nonetheless, several disadvantages. In particular, the need for a microwave cavity limits availability of reduction in the dimensions of the device. This limitation has been a main factor in the selection of rubidium 87 (hyperfine frequency=6.8 GHz) over rubidium 85 (hyperfine frequency=3.0 GHz), the cavity size required being larger for rubidium 85 than for rubidium 87. The microwave cavity also introduces restrictions on the type of material from which the resonance cell can be fabricated due to microwave losses which, in certain cases, can reduce the cavity Q to such an extent that it loses its microwave properties. Another disadvantage of the approach is the presence of a primary shift caused by the optical pumping radiation itself: it is termed the light shift. This light shift effect is of substantial importance in the case of optical pumping by means of lasers; thus, several approaches have been proposed to minimize the effect. However, all such approaches devolve about critical and often difficult adjustments of physical parameters of the resonance cell, pumping source and isotopic filter. Moreover, as mentioned, the problem is amplified when a laser is employed to accomplish the optical pumping. Another difficulty is the requirement for frequency multiplication to the resonance hyperfine frequency in order to observe the resonance signal.
It should also be mentioned that the type of state selection just described does not use the coherence property of the optical pumping light source and is generally termed intensity optical pumping because it relies only on the intensity of the source and not on its coherence, even in the case wherein a laser is used to accomplish the optical pumping.
The present invention employs laser radiation to accomplish the optical pumping and relies on the coherence property, not the intensity, of said laser radiation. The invention uses the phenomenon of Coherent Population Trapping, hereinafter (CPT), to prepare the atoms into a coherent superposition of energy states. No intensity optical pumping is used and the ensemble ground state populations are not altered by the phenomenon. The population of both ground levels remain equal. CPT has been known for many years (Alzetta et al., 1976). It is best described by reference to FIG. 2 which represents the lower levels of the cesium atom as the exemplary alkali metal; and to FIG. 3 which shows an experimental arrangement to observe the phenomena.
The cesium atom ensemble is contained in a cell and is exposed to two coherent radiation fields at angular frequencies .omega..sub.1 and .omega..sub.2, as shown in FIG. 2. The difference between .omega..sub.i and .omega..sub.2 corresponds to the cesium atom ground state angular hyperfine frequency .omega..sub.hf =2.pi..nu..sub.hf. The radiation fields can be the radiation produced by two lasers locked to each other or the sideband of a single laser modulated at a sub-harmonic frequency of the alkali atom hyperfine frequency. The effect of the two laser radiation fields is to produce a strong coherence in the ground state at the hyperfine frequency and inhibit all transitions to the excited state P. All atoms are trapped in the ground state, thus giving rise to the name Coherent Population Trapping. At exact resonance, .omega..sub.1 -.omega..sub.2 =.omega..sub.hf, no transitions take place from the ground state to the excited state, no energy is absorbed from the laser radiation by means of transitions and no atoms are excited to the P state. If the frequency of one laser radiation is scanned around the resonance frequency of the transition in question, a sharp decrease in the intensity of the fluorescence from the resonance cell, as measured at right angle to the laser propagation, is observed. A similar but opposite resonance effect in other words, a sharp increase, is observed in the transmission of the laser radiation through the resonance cell. This resonance line effect, observed both in transmission and in fluorescence, reflects all the properties of the ground state hyperfine resonance as observed in the intensity optical pumping approach and can be used as in prior intensity optical pumping technology for implementing a frequency standard. This resonance line effect has the obvious advantage of being detected directly on the laser radiation without the need of an exciting microwave radiation, thus avoiding the need for a microwave cavity.
Suggestions have been made for using the phenomenon in implementing a frequency standard and two important studies have been published in this connection (Cyr et al., 1993 using rubidium and Levi et al., 1997 using cesium). The theory has been given in several articles (Orriols, 1979, Vanier et al., 1989, Vanier et al. 1998). In the first article (Cyr et al., 1993), the CPT resonance signal is observed by means of a probe beam derived from the same frequency modulated laser used to create the population trapping phenomenon. This approach, although having some possible advantages regarding signal to noise ratio, entails substantial complexity in the experimental arrangement. In the second article (Levi et al., 1997), the signal is disclosed to be observed directly on the fluorescence from a glass cell containing the cesium atomic ensemble; it is shown in particular that the resonance line has very interesting properties for implementing a CPT frequency standard but the study reported is limited to basic properties and principles of the CPT phenomenon.
It is a principal object of this invention to provide a practical embodiment of an atomic frequency standard based on CPT using either or both of the fluorescence or light transmission line effects.
It is also an object of this invention to provide a novel resonance cell for implementing a compact atomic frequency standard based on the CPT phenomenon.
Another object of the present invention is to provide a novel method and apparatus for effectuating the CPT Phenomenon employing a VCSEL laser.
It is still another object of the invention to provide novel means of locking the laser frequencies and their difference to the atomic resonance lines.
Another object of the invention is to provide an optically pumped atomic frequency standard wherein the adverse light shift observed in the intensity pumping approach is avoided or at least substantially ameliorated.
An additional object of the invention is to provide an optically pumped atomic frequency standard wherein the known benefits of the use of a buffer gas or a mixture of such buffer gases are maintained to prevent Doppler broadening, wall relaxation and broadening by transit time effects, and to provide a small temperature coefficient.
Other objects and advantages of the present invention will, in part, be obvious and will, in part, appear hereinafter.